In one type of conventional outboard motor, a propeller is driven by a powerhead to propel a boat through water. Most large outboard motors of this type inject the exhaust gas stream under water in order to reduce engine noise and increase propulsive thrust.
In a typical configuration shown in FIG. 1, the gas exhausted from the powerhead 10 flows downwardly through an exhaust channel 12 and exits the motor rearwardly through the propeller 14. This type of motor is referred to as an exhaust-through-hub (ETH) motor.
Another type of conventional outboard motor has an axial-flow pump jet system driven by the powerhead. In a pump jet system, an impeller or rotor is mounted (e.g., spline fitted) directly on the propeller output shaft in place of the propeller. There are typically no modifications to the drive train, cooling or sealing components. A ducted housing surrounds the rotor. Such a system has the advantages of reducing hazards to swimmers in the vicinity of the motor, protecting the rotating elements from interference with and damage by foreign objects in the water, and improving the efficiency and performance of the propulsion system. Another benefit inherent with the pump jet is a directed jet of water that results in greater steering response.
U.S. Pat. No. 5,325,662 discloses a pump jet in which the exhaust gas discharged from the outboard motor is ducted downwardly through the central body of the motor and around a rotor shaft. An annular exhaust channel is formed in the rotor hub for receiving the exhaust gas and projecting it rearwardly of the motor. A cavity in the stator hub provides a plenum chamber for receiving the exhaust gas. Exhaust gas flows from the cavity of the stator hub to at least one hollow stator vane which serves as an exhaust pipe. In the case of multiple hollow stator vanes, the flow in the stator hub is split into multiple streams. Each stream of exhaust gas passes through a respective hollow stator vane. Discharge ports are formed in the stator housing for discharging exhaust gas into the water stream surrounding the stator housing. This arrangement will be referred to herein as an exhaust-through-vane (ETV) configuration.
The ETV configuration works well in practice. But the cross-sectional area of the hollow passages in the assemblage of hollow stator vanes is limited by practical considerations, with the result that the velocity of the gas streams exiting from the vanes must be several times greater than the velocity of the boat and pump jet through the water. There is a mismatch between boat velocity and exhaust gas velocity, so the exhaust gas stream must slow down, and in doing so, the exhaust gas stream "bushes out" and presents a significant added frontal area to the water stream, producing added drag. There is also a mismatch in the direction of the exhaust gas flow, which further adds to the frontal area of the gas stream, producing more drag.
Thus, there is a need for an improvement to an ETV-type pump jet which will minimize the mismatch in velocity and the mismatch in direction, thereby enhancing motor performance.